There has been a widespread use of various types of optical elements made of optical glasses, such as an imaging lens that is to be incorporated in a camera-equipped cellular phone and an optical pickup lens and a collimator lens that are to be incorporated in an optical disc device, and it has been increasingly demanded that such optical elements be produced with improved productivity at a reduced cost. Furthermore, as a glass molding method used for production of optical elements, there is known liquid-drop molding (direct precision press-molding). Liquid-drop molding is a molding method in which glass is allowed to drip through a nozzle, and drops of the dripping glass are directly received in a mold and then is pressed into a final shape. In this molding method, the temperature of the glass is controlled by controlling the temperature of the nozzle, and the size of the glass drops is controlled based on a relationship between the temperature and viscosity of the glass. Types of glass usable for this liquid-drop molding include, for example, those which are proposed in Patent Documents 1 to 3.
Patent Document 1 discloses types of glass having a refractive index (nd) of about 1.64 to 1.72, an Abbe number (vd) of 29 to 36, a glass deformation temperature (At) of 520° C. or lower, and a liquidus temperature (TL) of 900° C. or lower. Types of glass of Examples described in Patent Document 1 contain BaO having an increased effect of decreasing the liquidus temperature (TL) and further contain at least one of WO3 and Bi2O3, which have an effect of decreasing the glass deformation temperature (At) and a glass transition temperature (Tg). Presumably, it is thanks to effective actions of these ingredients that the decreased liquidus temperature (TL) and the decreased glass deformation temperature (At) and glass transition temperature (Tg) are realized.
In a case of performing liquid-drop molding of glass of the type containing WO3, however, when a mold made of tungsten carbide (WC) is used, due to a W ingredient contained in both of the glass and the mold, a reaction between the glass and the mold progresses, as a result of which the glass becomes fused to the mold to cause a mold life to be shortened. This renders this type of glass unsuitable for mass production. Furthermore, in a case of glass of the type containing Bi2O3, due to a low melting point of a Bi2O3 ingredient, a reaction between the glass and the mold progresses, so that, similarly to the above, the glass becomes fused to the mold to cause the mold life to be shortened. This renders this type of glass unsuitable for mass production. In Examples described in Patent Document 1, compositions having a refractive index (nd) of not less than 1.68 were found by actual measurement to have, at 100° C. to 300° C., a linear expansion coefficient (α) exceeding 130×10−7/° C. Presumably, this is attributable to a content of ZnO as small as 0% to 15% by weight.
Patent Document 2 discloses types of low-melting point glass having a refractive index (nd) of about 1.69 to 1.83, an Abbe number (vd) of 21 to 35, and a glass deformation temperature (At) of 570° C. or lower. These types of glass, however, all contain WO3, and thus when a mold made of tungsten carbide (WC) is used, a reaction between each of the types of glass and the mold progresses, as a result of which the each of the types of glass becomes fused to the mold to cause a mold life to be shortened. This renders these types of glass unsuitable for mass production.
Patent Document 3 discloses a type of glass having a refractive index (nd) of 1.6855 and an Abbe number (vd) of 34.7. This type of glass had, as actual measurement values, a glass transition temperature (Tg) of 476° C., a glass deformation temperature (At) of 519° C., a linear expansion coefficient (α) of 109×10−7/° C., and a liquidus temperature (TL) of 980° C. or higher. Presumably, the increased liquidus temperature (TL) is attributable to a content of a ZnO ingredient as excessive as 48.94 mol %. Due to the increased liquidus temperature (TL), it becomes likely that the glass is devitrified in a nozzle, which renders the glass unsuitable for mass production.